<p>According to neural oscillatory accounts, periodicity at the syllabic scale enhances speech comprehension through theta brain rhythms. Natural speech, however, is not strictly periodic and stronger periodicity, such as under conditions of fast speech, may hinder comprehension. Using magnetoencephalography, we investigate how natural variation in syllabic-level periodicity affects comprehension and auditory-motor coupling in brain areas related to temporal speech processing. We model speech periodicity and rate independently. Theta-band phase coupling between the posterior superior temporal gyrus (pSTG) and speech motor areas is assessed using Gaussian-Copula Mutual Information (GCMI). We find that faster syllabic rates and lower periodicity are associated with stronger coupling between the pSTG and inferior precentral gyrus, but also inferior frontal gyrus and supplementary motor areas. Comprehension improves with lower periodicity and declines at faster rates. The syllabic rate and periodicity moderate the coupling-comprehension relationship, possibly reflecting a complex interplay of lower-level auditory processing and higher-level prediction from the speech motor cortices. These findings suggest a sweet spot for natural, less periodic speech rhythms that support optimal processing and emphasize the necessity to investigate natural speech.</p>